1. Field of the Invention
This invention relates, in general, to printing and reproduction apparatus and, more specifically, to the registration of multiple image frames in electrophotographic copiers, printers, and other apparatus having synchronized image exposure and related processing.
2. Description of the Prior Art
In high speed electrostatographic reproduction apparatus, it is a common practice to employ photosensitive media in the form of an elongated photoconductive web (or a photoconductive coating on a drum) adapted to record transferable images. Such media moves in a path in operative relation with various electrophotographic process stations. Ultimately a transferable image is created and transferred to a receiver to produce a print or reproduction.
In making monochrome reproductions with an apparatus utilizing a uniformly charged photosensitive media, areas of uniform charge are exposed to light to form a charge pattern, or latent image frame. In the color reproduction arts, the image frame can be designed to correspond to one component in a set of related color separation images; several successive image frames on the media may thus constitute a set of component (separation) images which ultimately are used to provide a composite multicolor image. The latent images are developed with pigmented marking particles to form transferable images. Each image is transferred sequentially to a respective receiver member that may in turn be used as one of the several color separations for a composite multicolor reproduction. Alternatively, each image may be transferred directly to a single receiver to create a multichromatic (multicolor) print in one step.
In a web-based reproduction or printing apparatus, the web is typically supported by several freely turning rollers and driven by one drive roller. (In alternative reproduction apparatus, a driven drum assembly is substituted for the web and rollers). Because these driving assemblies are electro-mechanical systems, there is a tendency for the web (or drum) to vary in speed as it is driven. Moreover, because the web or drum is photosensitive and typically is exposed one line at a time by a laser beam or linear LED array, the writing (exposure) of each image frame must be positioned accurately and consistently on the web. Each latent image frame, after development, may be transferred to a receiver sheet and then fused. It has been contemplated that in one particularly important application, the receiver sheet may be specially designed and fabricated such that after receiving the toned and fused image, the sheet may be directly employed as a separation master in a multicolor xeroprinting machine. In an example of another application, each of the latent images would be developed with a respective colored toner and then transferred seriatim to a single receiver.
As a further example, it will be noted that a drum-based system may be found to operate similarly, in that the web may be replaced by a photosensitive drum of sufficient circumference to engage the electrophotographic process stations in the same manner as does the web. The drum is typically driven by a motor and exposed, for example, by a scanning laser beam or other exposure device to provide a latent image at the drum surface on a line-by-line sequence. For clarity, however, the background of the present invention is discussed with reference to a web-based system.
The separation images must be accurately registerable in order to obtain faithful multicolor reproductions. In such composite image applications, the transferable images generated from these "master" separations are aligned for accurate superimposed registration during the creation of a multicolor composite print. The composite print will suffer from undersirable color shift and fringing if the registration of the component images is not accurately established and maintained. Even in the relatively less-complex task of providing accent color images, wherein as few as two component images are used, image registration can be critical to providing an acceptable composite print. Thus, the registration of each image frame must be accurately established at the creation of the latent image, and carefully monitored and maintained thereafter. Otherwise, the resulting composite prints will be of inadequate or unusable quality.
Multicolor reproduction apparatus that produce composite color images, or other apparatus that produce color separations useable for high volume reproduction work, thus present strict registration requirements. The production of a misregistered separation set is costly in that any subsequently-generated composite image is inaccurate and the printing process must be halted while a new set of separations are made. In the color reproduction industry, such a waste of process time is significant and is to be avoided.
Moreover, in printers and scanners that use image data that is transferred to and from a digital memory, the image frame registration of component images in the digitized data stream is often precisely controlled. Yet, when such data is used to write images in an apparatus that cannot maintain such registration, this precision is compromised.
A means of registration of images on a photoconductor with transfer to a copy sheet has been described in U.S. Pat. No. 4,477,176 to Russel, wherein a roller is used to retain and move a copy sheet so as to transfer several image frames from a photosensitive member. Timing of this roller is controlled by a logic and control unit which in turn receives timing signals regarding web movement from a timing signal generator that senses regularly spaced perforations on the web. As indicated in U.S. Pat. No. 4,025,186 to Hunt, Jr. et al, a photosensitive web may be provided with two types of perforations, one comprising a series of closely spaced perforations for providing timing signals and the other identifying frame locations on the web for use in triggering a flash exposure. However, the high degree of registration necessary to generate some composite images is difficult to achieve due to inaccuracies in perforation shape or positioning and in the performance of related perforation sensing and counter circuitry.
U.S. Pat. No. 4,007,489, issued to Helmberger, discloses a method and apparatus for creating color copies of an original by an electrostatic charging process. The apparatus includes at each recording station a line of electrodes. A mark on the paper is sensed at each station and the deviation of the position of the mark from a desired position in the line direction determines which electrode will be used to start the recording of the line. In the direction of transport, the deviation of the position of the mark is sensed and the start of the line recording is accordingly varied in time. However, the charges must be deposited onto the reproduction carrier itself, by separate electrode assemblies, with one assembly at each of three recording systems.
U.S. Pat. No. 4,740,802, issued to Stuckey-Kauffman et al, discloses a method for determining the position of media in a system where images are applied to the media at subsequent stations. The method includes the steps of placing on the media at a first position tracking information consisting of a pattern of electrostatic charge to provide information with regard to the alignment of the media at the first station, and at the second station, detecting the pattern of electrostatic charge to determine the alignment of the media at the second station. However, the media typically consists of electrostatic paper that ultimately is used as the reproduction carrier and the step of placing charge on the paper is achieved by charged styli that are positioned in close proximity to the surface of the paper. Furthermore, the tracking information is applied continuously and thus is not particularly related to an individual image. The tracking information is thus used to determine the position of the media, with the apparent presumption that the image position is thereby indirectly ascertainable. The tracking information thus does not appear to be directly informative of the image position.